Java DNS解析问题-以Socket Client为例

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JDK 的InetSocketAddress被设计了为一个不可变的对象，在Socket等长连接使用时需要注意DNS映射关系更新问题。

问题描述

最近遇到一个zookeeper-client 更新DNS映射关系未生效问题，记录一下该问题的原因。

域名映射关系：

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127.0.0.1 test.lan-gl-42.dev

java应用程序

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-Dsun.net.inetaddr.ttl=15

Java Socket Client Code:

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@Test
   public void testSocket() {
       String host = "test.lan-gl-42.dev";
       int port = 9876;
       try (Socket socket = new Socket(host, port);
            ObjectOutputStream oos = new ObjectOutputStream(socket.getOutputStream())) {
           //write to socket using ObjectOutputStream
           System.out.println("Sending request to Socket Server,socket remote:" + socket.getRemoteSocketAddress().toString());
           while (true){
               Thread.sleep(10000);
               System.out.println("Sending request to Socket Server,socket remote:" + socket.getRemoteSocketAddress().toString());
           }
           //Thread.sleep(10000);
       } catch (IOException | InterruptedException  e) {
           e.printStackTrace();
       }catch (Exception e){
           e.printStackTrace();
       }
   }

执行日志： 

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 Sending request to Socket Server,socket remote:test.lan-gl-42.dev/127.0.0.1:9876
Sending request to Socket Server,socket remote:test.lan-gl-42.dev/127.0.0.1:9876
Sending request to Socket Server,socket remote:test.lan-gl-42.dev/127.0.0.1:9876
修改域名的映射关系为：

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10.224.17.250 test.lan-gl-42.dev

** socket连接的地址依然是 127.0.0.1 **

原因

Java进程及OS DNS解析过程： 图片来源 JVM会缓存DNS name lookups,它在指定的时间段内缓存IP地址，称为生存时间(time-to-live, TTL)

因为InetSocketAddress被设计了为一个不可变的对象（immutable object ）,Socket 对象实例的InetSocketAddress 对象，只在实例化对象时解析hostName与hostAddress的映射关系。

修复方法

可以借鉴 zookeeper 的 StaticHostProvider.java 调用next()方法在解析一次域名与IP地址的映射关系。

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public InetSocketAddress next(long spinDelay) {
        boolean needToSleep = false;
        InetSocketAddress addr;
        synchronized(this) {
            if (this.reconfigMode) {
                addr = this.nextHostInReconfigMode();
                if (addr != null) {
                    this.currentIndex = this.serverAddresses.indexOf(addr);
                    return this.resolve(addr);
                }

                this.reconfigMode = false;
                needToSleep = spinDelay > 0L;
            }

            ++this.currentIndex;
            if (this.currentIndex == this.serverAddresses.size()) {
                this.currentIndex = 0;
            }

            addr = (InetSocketAddress)this.serverAddresses.get(this.currentIndex);
            needToSleep = needToSleep || this.currentIndex == this.lastIndex && spinDelay > 0L;
            if (this.lastIndex == -1) {
                this.lastIndex = 0;
            }
        }

        if (needToSleep) {
            try {
                Thread.sleep(spinDelay);
            } catch (InterruptedException var7) {
                LOG.warn("Unexpected exception", var7);
            }
        }

        return this.resolve(addr);
    }

private InetSocketAddress resolve(InetSocketAddress address) {
        try {
            String curHostString = address.getHostString();
            List<InetAddress> resolvedAddresses = new ArrayList(Arrays.asList(this.resolver.getAllByName(curHostString)));
            if (resolvedAddresses.isEmpty()) {
                return address;
            } else {
                Collections.shuffle(resolvedAddresses);
                return new InetSocketAddress((InetAddress)resolvedAddresses.get(0), address.getPort());
            }
        } catch (UnknownHostException var4) {
            LOG.error("Unable to resolve address: {}", address.toString(), var4);
            return address;
        }
    }

扩展

JVM的DNS缓存时间可用通过一下参数配置:

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#globally, for all applications that use the JVM. Set networkaddress.cache.ttl in the $JAVA_HOME/jre/lib/security/java.security file
networkaddress.cache.ttl

#for your application only, set networkaddress.cache.ttl in your application’s initialization code:
sun.net.inetaddr.ttl

networkaddress.cache.ttl 是全局参数，所有使用该JVM的程序 sun.net.inetaddr.ttl JVM应用的配置参数，一般通过 -Dsun.net.inetaddr.ttl=10j进行配置

参考